Ichnogenic Megaporosity and Permeability in Carbonate Aquifers and Reservoirs: Definitions and Examples
Curran, H. Allen; Cunningham, Kevin J.
Biogenic megaporosity in sedimentary deposits (readily visible without magnification) typically has body-fossil moldic or ichnogenic origin. We consider ichnogenic megaporosity as pores greater than 4 mm associated with either burrow- or rhizolith-dominated ichnofabrics. Dominant bioturbators in shallow-marine environments typically include thalassinidean crustaceans and polychaetes. Callianassid shrimp commonly dominate the deep-tier fauna in carbonate and siliciclastic, sandy, shallow-marine settings; when fossilized their thickly lined, pelleted burrows (cm-scale outside diameters) are assigned to the ichnogenus Ophiomorpha. In the mostly Pleistocene carbonate rocks of the Biscayne aquifer of south Florida, Ophiomorpha is well lithified and burrows form a rigid framework, with interburrow macroporosity developed by matrix transport and dissolution. Tubular burrows commonly remain open or with fill washed clean in the vadose zone or removed by dissolution, resulting in intraburrow megaporosity. Both megaporosity types greatly enhance aquifer porosity and permeability, commonly in combination.
Lower Cretaceous carbonates of the Edwards-Trinity aquifer system in central Texas also manifest ichnogenic megaporosity in zones dominated by Thalassinoides, unlined, Y-branched burrows (cm-scale diameters). Again, both inter- and intraburrow megaporosity result from dissolution of carbonate matrix surrounding lithified burrow fills or fabric-selective leaching and removal of burrow fill material or both. Horizons with Thalassinoides-related intraburrow macroporosity in the Edwards-Trinity system have Lattice Boltzmann-calculated permeabilities within the range of Ophiomorpha-related permeabilities measured from the Biscayne aquifer. As a third example, outcropping Upper Pleistocene regressive carbonate eolianites throughout the Bahamas often exhibit horizons with dense occurrences of rhizoliths, developed by diagenetic activity of plant roots interacting with host sediment. Branching rhizoliths can form sturdy frameworks with inter-rhizolith megaporosity commonly resulting as matrix material is removed by sediment transport and dissolution. Intra-rhizolith megaporosity can occur when rhizolith tubules have fill material removed, although this process appears subordinate. Examples of rhizolith megaporosity also have been documented from the Biscayne aquifer system, and this form of megaporosity likely is widespread in carbonate eolianites elsewhere.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013